Separator-integrated electrode and nonaqueous electrolyte secondary battery

The present invention relates to separator-integrated electrodes and nonaqueous electrolyte secondary batteries including separator-integrated electrodes.

Recent years have seen a significant reduction in the size and weight of mobile information terminals such as cellular phones and laptop computers, and there is a need for a battery with a higher capacity as their power supply. Although nonaqueous electrolyte secondary batteries, such as lithium (Li) ion batteries, which have particularly high energy density among secondary batteries, have increasingly high capacities, the above need has yet to be satisfied. Recently, nonaqueous electrolyte secondary batteries have been used in a broader range of applications due to their characteristics. For example, nonaqueous electrolyte secondary batteries have increasingly been used not only in mobile applications such as cellular phones, portable computers, PDAs, and portable music players, but also in medium to large battery applications, including power applications such as electric tools, electric bicycles, electric vehicles (EVs), and hybrid electric vehicles (HEVs and PHEVs) and storage applications such as backup power supplies and power storage systems.

With the increasing applications of nonaqueous electrolyte secondary batteries, nonaqueous electrolyte secondary batteries having an increasingly wide variety of structures have been developed. Examples of such nonaqueous electrolyte secondary batteries include cylindrical batteries including a rolled electrode assembly composed of electrodes and a separator rolled together, rectangular batteries including a rolled electrode assembly pressed and inserted into a rectangular casing or covered with a laminate casing, rectangular batteries including an electrode assembly composed of electrodes and separators stacked on top of each other and inserted into a rectangular casing or covered with a laminate casing, and button batteries.

The development of nonaqueous electrolyte secondary batteries of various shapes has complicated the manufacturing processes of nonaqueous electrolyte secondary batteries. This leads to decreased productivity, including increased manufacturing costs of nonaqueous electrolyte secondary batteries and an extended lead time. Accordingly, it is essential to develop a technique for simplifying the manufacturing processes of an increasingly wide variety of nonaqueous electrolyte secondary batteries.

Polyolefin fine porous films, which are used as separators in nonaqueous electrolyte secondary batteries, increase the cost of battery materials and involve a complicated manufacturing process and an extended lead time due to the need for the steps of stacking and rolling together with electrodes. Accordingly, the inventors have made various attempts to develop a battery including no conventional polyolefin fine porous film, i.e., a separatorless battery.

Conventional nonaqueous electrolytes secondary batteries include a separator made of an insulating porous material through which lithium ions pass between positive and negative electrode plates. The separator is a fine porous film of a polyolefin such as polyethylene or polypropylene. To develop a nonaqueous electrolyte secondary battery including no such separator, i.e., a separatorless battery, a porous layer of a low-cost material must be directly formed on a surface of an electrode to replace a polyolefin fine porous film.

PTL 1 below discloses an invention related to a nonaqueous electrolyte secondary battery that includes a negative electrode including a current collector and a negative active material coating layer disposed thereon, a positive electrode including a current collector and a positive active material coating layer disposed thereon, a separator, and a nonaqueous electrolyte. A porous protective film having a thickness of 0.1 to 200 μm and containing a resin binder and fine solid particles of alumina or silica powder having particle sizes in the range from 0.1 to 50 μm is formed on a surface of the positive or negative active material coating layer. PTL 2 below discloses an invention related to a separator-electrode-integrated storage element for electrochemical devices that includes an electrode and a separator integrated with a surface thereof. The separator is a porous layer containing fibers formed by electrospinning.

Patent Literature

PTL 1: Japanese Patent No. 3371301

PTL 2: Japanese Published Unexamined Patent Application No. 2010-225809

Technical Problem

According to the invention disclosed in PTL 1 above, the porous protective film formed on the surface of the positive or negative active material coating layer provides a nonaqueous electrolyte secondary battery with a reduced risk of an internal short circuit due to freed active material. According to the invention disclosed in PTL 2 above, the fiber-containing porous layer formed on the surface of the active material layer provides a separator-electrode integrated storage element for electrochemical devices with reduced internal resistance that includes a thinner porous layer with sufficient mechanical strength.

The results of experiments conducted by the inventors, however, have revealed that separatorless nonaqueous electrolyte secondary batteries including a positive or negative electrode having a porous layer made of solid particles or containing fibers having diameters of less than 1 μm on a surface of an active material layer suffer an internal short circuit due to precipitation of dendrites of metallic lithium on the surface of the active material layer during initial charge.

A porous layer made of solid particles or containing fibers having diameters of less than 1 μm tends to have uneven texture if the porous layer is thin, i.e., several tens of micrometers thick. This results in a distribution of areas with low resistance to passage of lithium ions. During initial charge, the lithium ion concentration increases in those areas with low resistance to passage of lithium ions. As a result, dendrites of metallic lithium precipitate on the surface of the active material layer and cause a physical short circuit between the positive and negative electrodes.

The present invention provides a separator-integrated electrode including a porous layer with a reduced risk of a short circuit between positive and negative electrodes due to precipitation of metallic lithium on a surface of a positive or negative active material layer and also provides a nonaqueous electrolyte secondary battery including such a separator-integrated electrode.

Solution to Problem

A separator-integrated electrode according to the present invention includes a current collector, an active material layer formed on the current collector, and a porous layer formed on the active material layer. The porous layer contains inorganic particles and resin fibers.

Because the porous layer formed on the surface of the active material layer in the separator-integrated electrode according to the present invention contains inorganic particles and resin fibers, the porous layer has a more even texture and a smaller area with low resistance to passage of lithium ions than conventional porous layers. The separator-integrated electrode according to the present invention is therefore resistant to precipitation of dendrites of metallic lithium due to increased lithium ion concentration in the low-resistance area. Thus, the nonaqueous electrolyte secondary battery including the separator-integrated electrode according to the present invention has a reduced risk of a short circuit between electrodes and improved reliability and electrochemical characteristics.

The resin fibers forming the separator-integrated electrode according to the present invention are preferably a fibrous resin material having diameters of from one to several hundreds of nanometers, although a fibrous resin material having diameters of several micrometers can also be used. The separator-integrated electrode according to the present invention is applicable to either a positive electrode including a positive current collector and a positive active material layer formed on a surface thereof or a negative electrode including a negative electrode current collector and a negative active material layer formed on a surface thereof.

In the separator-integrated electrode according to the above aspect, the porous layer preferably includes a first porous layer formed on the active material layer and containing the inorganic particles and a second porous layer formed on the first porous layer and containing the resin fibers.

The thus-configured porous layer, in which the first porous layer containing the inorganic particles and the second porous layer made of the resin fibers are formed on the surface of the active material layer, exhibits good characteristics based on the difference between the materials for the individual porous layers.

In the separator-integrated electrode according to the above aspect, the relationship 0.5≦x/(x+y)≦0.95 is preferably satisfied, where x is the average thickness of the second porous layer, and y is the average thickness of the first porous layer.

If the value of x/(x+y), which is the thickness x of the porous layer made of the resin fibers divided by the sum (x+y) of the thickness of the porous layer containing the inorganic particles and the thickness of the porous layer made of the fibers, is less than 0.5, the porous layer containing the inorganic particles and the porous layer made of the fibers have an uneven resistance distribution, which tends to result in an internal short circuit. If the value of x/(x+y) is more than 0.95, the porous layer made of the resin fibers has a smaller distribution of areas with low resistance to passage of lithium ions; however, such a value of x/(x+y) is undesirable because of increased internal resistance.

In the separator-integrated electrode according to the above aspect, the resin fibers forming the second porous layer is preferably made of at least one polymeric material selected from polypropylene, polyethylene, polystyrene, polyethylene oxide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, poly-m-phenylene terephthalate, poly-p-phenylene isophthalate, polyvinylidene fluoride, polytetrafluoroethylene, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene) copolymer, poly(tetrafluoroethylene-hexafluoropropylene) copolymer, polyvinyl chloride, polyvinylidene chloride-acrylate copolymer, polyacrylonitrile, polyacrylonitrile-methacrylate copolymer, polycarbonates, polyarylates, polyester carbonates, polyamides, aramids, polyimides, polycaprolactone, polylactic acid, polyvinyl alcohol, polyglycolic acid, collagen, polyhydroxybutyric acid, polyvinyl acetate, polypeptides, and copolymers thereof.

A second porous layer formed using such resin fibers has good electrical insulation properties and porosity. A nonaqueous electrolyte secondary battery including such a second porous layer is more resistant to precipitation of dendrites of lithium and thus has a reduced risk of an internal short circuit.

In the separator-integrated electrode according to the above aspect, the inorganic particles in the first porous layer are preferably at least one material selected from titania (excluding anatase), alumina, zirconia, and magnesia.

These inorganic particles have good electrical insulation properties, low cohesion, and good dispersibility in solvent and thus facilitate formation of a homogeneous first porous layer.

In the separator-integrated electrode according to the above aspect, the resin fibers forming the second porous layer is preferably formed by electrospinning.

Electrospinning facilitates shape control and elongation of the resulting fibers and permits a wide selection of resin materials as compared to other processes. In addition, electrostatic force allows the fibers to readily enter pores in the insulating first porous layer. This stabilizes the formation of the second porous layer and increases the adhesion strength between the first and second porous layers.

A nonaqueous electrolyte secondary battery according to the present invention includes one of the above separator-integrated electrodes and a counter electrode paired with the separator-integrated electrode. The counter electrode includes a current collector and a counter active material layer formed on a surface of the current collector. The counter active material layer is disposed opposite and in contact with the second porous layer of the separator-integrated electrode. The first and second porous layers of the separator-integrated electrode are impregnated with a nonaqueous electrolyte solution. The nonaqueous electrolyte secondary battery may include a separate separator.

The nonaqueous electrolyte secondary battery according to the present invention has a reduced risk of a short circuit between the electrodes and thus has high reliability.

FIG. 1 is a schematic sectional view of a nonaqueous electrolyte secondary battery according to an embodiment.

FIG. 2 is a schematic sectional view of a nonaqueous electrolyte secondary battery according to a modification.

An embodiment of the present invention will now be described in detail. The following embodiment is given by way of illustration for a better understanding of the technical idea of one aspect of the present invention and is not intended to limit the present invention. Rather, various modifications are possible without departing from the technical idea set forth in the claims. The separator-integrated electrode according to the present invention is applicable to either a positive electrode or a negative electrode. The following description will be directed primarily to a positive electrode.

A nonaqueous electrolyte secondary battery 10 according to this embodiment will now be described with reference to FIG. 1 . The nonaqueous electrolyte secondary battery 10 according to this embodiment includes a positive electrode 12 configured as a separator-integrated electrode and a negative electrode 14 disposed on the separator-integrated positive electrode 12 .

The positive electrode 12 includes, in sequence, a positive current collector 22 , a positive active material layer 24 , a first porous layer 26 containing inorganic particles, and a second porous layer 28 containing a resin material. The negative electrode 14 includes a negative current collector 32 and a negative active material layer 34 disposed thereon.

Specifically, the positive current collector 22 of the positive electrode 12 is made of aluminum or aluminum alloy. The positive active material layer 24 is formed on a surface of the positive current collector 22 . The first porous layer 26 is formed on a surface of the positive active material layer 24 facing away from the positive current collector 22 . The second porous layer 28 is formed on a surface of the first porous layer 26 facing away from the positive active material layer 24 .

Examples of positive active materials capable of absorbing and releasing lithium ions for the positive active material layer 24 include lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMn 2 O 4 and LiMnO 2 ), lithium nickel oxide (LiNiO 2 ), lithium nickel cobalt oxide (LiNi x Co 1-x O 2 (where x=0.01 to 0.99)), lithium nickel manganese cobalt oxide (LiNi x Mn y Co z O 2 (where x+y+z=1)), and lithium iron phosphate (LiFePO 4 ), which can be used alone or in a mixture. Lithium cobalt oxides containing other metal elements such as zirconium, magnesium, and aluminum can also be used.

The first porous layer 26 is made of, for example, inorganic particles and a resin material. Examples of inorganic particles include titania (TiO 2 ), alumina (Al 2 O 3 ), zirconia (ZrO 2 ), and magnesia (MgO). The titania is preferably rutile. Rutile is more resistant to insertion and extraction of lithium ions and thus is more resistant to, for example, absorption of lithium ions and electrical conduction due to the ambient atmosphere (potential) than anatase. This reduces the decrease in capacity and the risk of a short circuit. The inorganic particles preferably have an average particle size of 1 μm or less. For reasons of dispersibility in slurry, it is particularly prepared to use inorganic particles surface-treated with aluminum (Al), silicon (Si), titanium (Ti), or the like.

The resin material for the first porous layer 26 may be a water-insoluble resin or a water-soluble resin. Examples of water-insoluble resins include polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyvinylidene fluoride, polyacrylonitrile, polyimides, polyamides, polyamide-imides, and polytetrafluoroethylene. Examples of water-soluble resins include polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyvinyl methyl ether, and polyethylene glycol.

The resin material for the first porous layer 26 is preferably dissolved in a solvent different from the solvent used in the positive active material slurry for the positive active material layer. Thus, if the first porous layer 26 is formed on a surface of a positive active material layer formed from a positive active material slurry prepared using N-methyl-2-pyrrolidone (NMP) as a solvent, the resin material for the first porous layer 26 is preferably dissolved in water.

Examples of solvents for dispersion of the inorganic particles and dissolution of the resin material to form the first porous layer 26 include organic solvents such as N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), hexamethylphosphoric triamide (HMPA), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and γ-butyrolactone. Any of these and other solvents for dispersion of the inorganic particles and dissolution of the resin material may be selected depending on the solubility of the resin material.

The first porous layer 26 can be formed on the surface of the positive active material layer 24 by applying a slurry prepared by dispersing the inorganic particles and dissolving the resin material in a solvent to the surface of the positive active material layer 24 or by other processes such as gravure coating, spray coating, die coating, roller coating, dip coating, and screen printing.

The second porous layer 28 is made of fibers. Examples of resin materials for the fibers include polymeric materials such as polypropylene, polyethylene, polystyrene, polyethylene oxide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, poly-m-phenylene terephthalate, poly-p-phenylene isophthalate, polyvinylidene fluoride, polytetrafluoroethylene, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene) copolymer, poly(tetrafluoroethylene-hexafluoropropylene) copolymer, polyvinyl chloride, polyvinylidene chloride-acrylate copolymer, polyacrylonitrile, polyacrylonitrile-methacrylate copolymer, polycarbonates, polyarylates, polyester carbonates, polyamides, aramids, polyimides, polycaprolactone, polylactic acid, polyvinyl alcohol, polyglycolic acid, collagen, polyhydroxybutyric acid, polyvinyl acetate, polypeptides, and copolymers thereof. These resin materials may be used alone or in a mixture. These resin materials are illustrative only, and other resin materials may be used for the fibers.

The second porous layer 28 , which is made of fibers, can be formed on the surface of the first porous layer 26 by known processes such as electrospinning, self-assembly, and phase separation. Electrospinning is preferred for its productivity and stability. Electrospinning is a process in which a high voltage is applied between a spinning solution stored in a container and a collector electrode on the target side to force the spinning solution out of the container and deposit it on the target in the form of fine charged fibers. In this embodiment, the second porous layer 28 may be formed on the surface of the first porous layer 26 using the positive electrode current collector 22 as the collector electrode.

The second porous layer 28 may be formed by electrospinning as follows. The resin material is first mixed and dispersed in a solvent to prepare a spinning solution. The resin material may be mixed and dispersed in the solvent in any manner, for example, using a planetary mixer, a homomixer, a pin mixer, a kneader, or a homogenizer. During the mixing and dispersion of the resin material in the solvent, various additives such as inorganic particles, dispersants, surfactants, stabilizers, and crosslinkers may be added if necessary.

The spinning solution is supplied to a nozzle and is forced out of the nozzle while being electrostatically charged by an electric field. The charged spinning solution is electrically attracted by the electrode on the collector side and is stretched into a fiber. In electrospinning, as the resulting fiber flies toward the collector, the fiber is accelerated by the electric field and is electrically stretched into a thinner fiber. At the same time, the fiber becomes thinner as the solvent evaporates and is split into even thinner fibers by electrical repulsion due to increased electrostatic density. The voltage applied to generate the electric field is preferably, but not necessarily, about 5 to 50 kV. The polarity of the applied voltage may be either positive or negative. The resulting fiber layer may be pressed to adjust the porosity of the fiber layer.

Examples of solvents for the spinning solution include methanol, ethanol, 1-propanol, 2-propanol, hexafluoroisopropanol, tetraethylene glycol, triethylene glycol, dibenzyl alcohol, 1,3-dioxolane, 1,4-dioxane, methyl ethyl ketone, methyl isobutyl ketone, methyl n-hexyl ketone, methyl n-propyl ketone, diisopropyl ketone, diisobutyl ketone, acetone, hexafluoroacetone, phenol, formic acid, methyl formate, ethyl formate, propyl formate, methyl benzoate, ethyl benzoate, propyl benzoate, methyl acetate, ethyl acetate, propyl acetate, dimethyl phthalate, diethyl phthalate, dipropyl phthalate, methyl chloride, ethyl chloride, methylene chloride, chloroform, o-chlorotoluene, p-chlorotoluene, chloroform, carbon tetrachloride, 1,1-dichloroethane, 1,2-dichloroethane, trichloroethane, dichloropropane, dibromoethane, dibromopropane, methyl bromide, ethyl bromide, propyl bromide, acetic acid, benzene, toluene, hexane, cyclohexane, cyclohexanone, cyclopentane, o-xylene, p-xylene, m-xylene, acetonitrile, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, pyridine, and water. These solvents may be used alone or in a mixture. These solvents are illustrative only, and other solvents may be used for the spinning solution.

Examples of inorganic materials for the spinning solution include oxides, carbides, nitrides, borides, silicides, fluorides, and sulfides. For reasons of the properties of the fibers to be manufactured, such as heat resistance and workability, it is preferred to use oxides as the inorganic material for the spinning solution.

Examples of oxides for the spinning solution include Al 2 O 3 , SiO 2 , TiO 2 , Li 2 O, Na 2 O, MgO, CaO, SrO, BaO, B 2 O 3 , P 2 O 5 , SnO 2 , ZrO 2 , K 2 O, Cs 2 O, ZnO, Sb 2 O 3 , As 2 O 3 , CeO 2 , V 2 O 5 , Cr 2 O 3 , MnO, Fe 2 O 3 , CoO, NiO, Y 2 O 3 , Lu 2 O 3 , Yb 2 O 3 , HfO 2 , and Nb 2 O 5 . These oxides may be used alone or in a mixture. These oxides are illustrative only, and other oxides may be used for the spinning solution.

The proportions of the solvent and the solute in the spinning solution vary depending on the types of solvent and solute selected. Preferably, the solvent is used in an amount of from about 60% to about 98% by mass, and the solute is preferably used in an amount of from 20 to 40% by mass.

The negative current collector 32 of the negative electrode 14 is made of copper or copper alloy, and the negative active material layer 34 is disposed thereon. Examples of negative active materials capable of absorbing and releasing lithium ions for the negative active material layer 34 include carbonaceous materials such as graphite, non-graphitizable carbon, and graphitizable carbon; titanium oxides such as LiTiO 2 and TiO 2 ; metalloid elements such as silicon and tin; silicon oxides (SiO x , where 0.5≦x<1.6); and tin-cobalt alloys.

10 , 10 A electrode assembly

12 separator-integrated electrode

14 negative electrode

18 second porous layer

22 positive current collector

24 positive active material layer

26 first porous layer

28 second porous layer

32 negative current collector

34 negative active material layer

40 electrode assembly

42 separator-integrated electrode

44 positive electrode